Re-focus public API on Address & Mailbox aka short-lived Context

[thomaseizinger]

As part of doing #122, I started to form the following vision:

• Chan should be the core channel implementation of the library, Move channel logic into Chan #122 gets us most of the way there
• Sender and Receiver only really add an opinionated interfaces on top of it, plus reference counting
• Neither Sender nor Receiver are publicly exposed, meaning I think they don't actually have to exist
• Sender can be completely inlined into Address, removing an entire layer of indirection
• Receiver can be promoted to a type called Mailbox that could be publicly exposed
• Context would be something that is constructed only temporarily for the invocation of a handler (I think we discussed this idea at some point)

Putting all of this together, we could realise an API like this:

let (mailbox, address) = Mailbox::bounded(5);

tokio::spawn(xtra::run(mailbox, MyActor::new()));

With run being implemented more or less like what we have today.

[Restioson]

I think this makes sense!